An electrocardiogram, or "ECG", is a graphical record of the electric potentials produced by activity of the heart. In the same way that heart activity is cyclical, the electrical potentials produced by activity of the heart are also cyclical, and measurement thereof has resulted in the well known characteristic ECG waveform. It is known that the portions of this waveform correspond to different events which together form the cyclical activity of the heart, and the known, basic P, Q, R, S, T, U waves are so-called, due to six commonly identified transition points on an ECG waveform; portions of the waveform bounded by these points being understood as representing activities of specific intra-heart organs. By way of example, the QRS portion (i.e. that portion of the waveform defined between the QRS nodes), known also as the QRS complex, occurs at the time that the ventricles of the heart contract, or "systole." The so-called "T wave" is caused by repolarization of the electrical system following contraction, or "diastole." It is understood that changes observed in any of these waves can be used for various cardio-diagnostic purposes, by way of example, the S-T elevation is taken as one of the classic signs of heart infarcation.
It is, in fact, known that current interpretation of the electrocardiogram is very limited, due to the fact that the heart is known to be a very complex organ, and each operating cycle thereof is not limited merely to a small number of events, such as the electrocardiogram might indicate. The occurrence of this very large number of heart events is well known, and, by way of example, is discussed in a book entitled CLINICAL SCALAR ELECTROCARDIOGRAPHY; by Lipman, Massie and Kleiger; and bearing Library of Congress Catalog Card Number 72-188577.
It is further known to extract useful information concerning certain heart organ functions by assessing variability of data contained therein. An indication of the state of the art is provided by U.S. Pat. No. 5,188,116 and 5,419,338.
The '116 publication, entitled "Electrocardiographic Method and Device," is directed to method and apparatus for detecting heart disease from an electrocardiogram (ECG). The method comprises the steps of acquiring the ECG signals, correcting for signal variability caused by breathing and calculating the level of remaining variability due to myocardial function. The apparatus comprises a signal input system, a storage system, a microprocessor and an output system. The microprocessor has program logic for processing signal data in accordance with the method
The '338 publication, entitled "Autonomic Nervous System Testing by Bi-Variate Spectral Analysis of Heart Period and QT Interval Variability," discloses apparatus and a method for testing the autonomic nervous system of a mammal through bi-variate analysis of heart Period (RR) and QT interval variability obtained through data processing of electrocardiographic data. The method and apparatus enable both the parasympathetic and sympathetic controls on the heart to be evaluated for imbalances therebetween, which may indicate a predisposition for sudden cardiac death.
Neither of the above methods provides an overall approach which enables the determination of the overall state of the heart of a subject relative to a norm, the mapping of various heart functions, and the determination of a trend, such that even if a subject is seen to have a serious heart condition, it is possible to establish whether his condition is improving or deteriorating.